Optical system for calibration and control of an optical fiber switch

ABSTRACT

Methods of calibrating and operating optical switches as well as optical switches in which the orientations of mirrors are measured and controlled using control light beams and position sensing detectors are described. The present invention may provide high resolution control of a plurality of mirrors in an optical switch and thus allow the optical switch to cross-connect a large number of input and output ports with a low insertion loss.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to the following co-filed, commonlyassigned, U.S. Patent Applications: Attorney Docket No. M-10967 US,Attorney Docket No. M-11419 US, Attorney Docket No. M-11501 US, AttorneyDocket No. M-11502 US, and Attorney Docket No. M-11745 US, all of whichare incorporated herein by reference. This application is also relatedto U.S. Patent Application Serial No. 09/779,189 entitled “AMicroelectromechanical Mirror,” filed Feb. 7, 2001, assigned to theassignee of the present invention, and incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to optical fiber cross-connect switching.

2. Description of the Related Art

As optical fiber progressively supplements and replaces metal wire asthe backbone of telecommunications networks, the switches that routeoptical signals have emerged as a significant bottleneck. Transmissionsystems move information as optical photons but the switching systemsand so-called cross-connect fabrics that switch, route, multiplex, anddemultiplex optical signals have generally been electronic. Electronicswitching requires light to be converted to an electronic signal to passthrough the switch and then be reconverted to light in a process termedoptical-electronic-optical (OEO) conversion that introduces both timedelay and cost.

There is great interest in the telecommunications industry, therefore,in developing all optical switching to avoid the necessity of multipleOEO conversions. As described, for example, by Bishop et al. inScientific American (January, 2001, pp. 88-94), all optical switchesbased on a number of underlying technologies including Micro ElectroMechanical Systems (EMS) tilting mirrors, thermo-optical devices,bubbles formed by inkjet printing heads, and liquid crystals, have beenproposed. Optical fiber switches based on MEMS mirrors are particularlyattractive because they can incorporate very large scale integratedcircuits and can be robust, long-lived, and scalable.

An optical fiber switch described in U.S. Pat. No. 5,960,132 to Lin, forexample, includes an array of hinged MEMS mirrors, each of which can berotated about its hinge between a reflective state and a non-reflectivestate. An array of N² such mirrors is required to switch signals carriedby N input optical fibers from one to another of N output opticalfibers. Unfortunately, N² scaling results in unmanageably complexdevices for large N.

Another optical fiber switch described in Bishop et al., cited above, aswell as in Bishop et al., Photonics Spectra (March 2000, pp. 167-169)includes an array of MEMS mirrors disposed on a single surface. Eachmirror tilts independently to direct light received from an array ofinput/output optical fibers to any other mirror and thus to anyinput/output fiber. This optical fiber switch does not appear to includeoptical diagnostics which would enable active closed-loop opticalfeedback control of the mirror orientations or allow input presencedetection.

Optical fiber switches having a low insertion loss and capable ofcross-connecting large numbers of input and output fibers would furtherthe development of fiber optic telecommunications networks.

SUMMARY

The present invention provides methods of calibrating and operatingoptical switches. A method of calibrating an optical switch including afirst port, a second port, a first mirror and a second mirror inaccordance with the present invention includes directing a first controlbeam of light onto the first mirror and directing a second control beamof light onto the second mirror. The method also includes controlling anorientation of the first mirror such that the first mirror reflects thefirst control beam of light to a predetermined position on a firstposition sensing detector, and controlling an orientation of the secondmirror such that the second mirror reflects the second control beam oflight to a predetermined position on a second position sensing detector.A third beam of light (e.g., at a telecommunications wavelength)incident on the first mirror from the first port is thereby directed tothe second mirror and thence to the second port. The method furtherincludes controlling the orientations of the first mirror and the secondmirror to maximize the intensity of the third beam of light coupled intothe second port, and recording signals determined from outputs providedby the first and second position sensing detectors.

In one implementation, the calibration method further includescontrolling the orientations of the first and second mirrors to minimizea reflection of the third beam of light from, for example, the secondport. The calibration method may also include directing a third controlbeam of light (fourth beam) onto a third position sensing detector viathe first mirror and the second mirror, and recording a signaldetermined from outputs provided by the third position sensing detector.

This calibration method, which is typically performed only once in thefactory after assembly of the optical switch, allows highly accuratecontrol of the mirrors in the optical switch during subsequent switchingoperations in the field.

A method of operating an optical switch including a first mirror and asecond mirror in accordance with an embodiment of the present inventionincludes directing a first control beam of light onto the first mirror,and controlling an orientation of the first mirror such that the firstmirror reflects at least a portion of the first control beam of light toa predetermined position on a first position sensing detector. Themethod also includes directing a second control beam of light onto thesecond mirror, and controlling an orientation of the second mirror suchthat the second mirror reflects at least a portion of the second controlbeam of light to a predetermined position on a second position sensingdetector. The method further includes directing a third control beam oflight onto the second mirror via the first mirror, and controlling anorientation of the first mirror, of the second mirror, or of bothmirrors such that at least a portion of the third control beam of lightis directed to a predetermined position on a third position sensingdetector. In one implementation, an orientation of at least one of thefirst and second mirrors is controlled with an angular resolution betterthan about 0.005°.

The first, second, and third predetermined positions referred to in theoperation method may be determined, for example, by the calibrationmethod described above.

The first, second, and third control light beams utilized in theoperation method are separate from the light beams being routed throughand switched by the optical switch. Hence, the mirrors in the opticalswitch can be aligned to couple signal (e.g., telecommunications) lightbeams input through particular input ports to particular output portsbefore those signal light beams are provided to the optical switch.Moreover, the orientations of the mirrors may be controlled withoutsampling the signal light beams, thus reducing the insertion loss of theoptical switch.

The present invention also provides optical switches in which theorientations of mirrors are measured and controlled using control lightbeams and position sensing detectors. In one embodiment, an opticalswitch includes a plurality of mirrors, a light source located toilluminate the plurality of mirrors, a plurality of position sensingdetectors, and an imaging system located to image the plurality ofmirrors at an image plane spaced apart from the detectors. Signalsprovided by the position sensing detectors correspond to orientations ofcorresponding mirrors. This arrangement allows the orientations of theplurality of mirrors to be measured and controlled with, for example, aresolution of better than about 0.04° over a range of greater than about20° (about 9 bit accuracy).

In another embodiment, an optical switch includes a first mirror, asecond mirror, a light source, and a position sensing detector locatedto detect light output by the first light source and reflected by bothof the first mirror and the second mirror. A signal provided by thedetector corresponds to the orientations of the first mirror and thesecond mirror. This arrangement allows the orientations of the first andsecond mirrors to be measured and controlled with, for example, aresolution of better than about 0.005° over a range of greater thanabout 0.15° (about 5 bit accuracy). If the optical switch also includeslight sources, imaging systems, and position sensing detectors asdescribed in the previous embodiment, then the orientations of themirrors may be measured and controlled with, for example, a resolutionbetter than about 0.005° over a range of greater than about 20° (about12 bit accuracy).

In another embodiment, an optical switch includes a first plurality ofmirrors, a second plurality of mirrors, a light source and a pluralityof position sensing detectors. Each detector is located to detect lightoutput by the light source, reflected by one of the first plurality ofmirrors, and reflected by one of the second plurality of mirrors.Signals provided by the detectors correspond to the orientations of thefirst plurality of mirrors and the second plurality of mirrors. In oneimplementation, the orientations of the mirrors in the first and secondpluralities of mirrors are measured and controlled with a resolution ofbetter than about 0.005° over a range of greater than about 0.15° (about5 bit accuracy). If the optical switch also includes light sources,imaging systems, and position sensing detectors as described above, thenthe orientations of the mirrors may be measured and controlled with, forexample, a resolution better than about 0.005° over a range of greaterthan about 20° (about 12 bit accuracy).

The high resolution control of mirror orientations achievable in opticalswitches provided by embodiments of the present invention reduces theinsertion loss of the optical switches. Consequently, optical fiberswitches may be controlled in accordance with embodiments of the presentinvention to cross-connect more than a thousand input ports to more thana thousand output ports with an insertion loss of less than, forexample, about 3 decibels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an optical switch in accordancewith an embodiment of the present invention.

FIG. 2 is a schematic illustration of an optical switch in accordancewith another embodiment of the present invention.

FIG. 3 is a schematic illustration of an optical switch including inputand output sensors in accordance with an embodiment of the presentinvention.

FIG. 4 is a schematic illustration of an optical switch includingmonitor channels in accordance with an embodiment of the presentinvention.

FIG. 5 is a schematic illustration of optical paths of control lightbeams in an optical switch in accordance with an embodiment of thepresent invention.

FIG. 6 is a schematic illustration of optical paths of control lightbeams in an optical switch in accordance with another embodiment of thepresent invention.

FIG. 7 is a schematic illustration of optical paths of control lightbeams in an optical switch in accordance with another embodiment of thepresent invention.

FIG. 8 is a schematic illustration of optical paths of reference lightbeams in an optical switch in accordance with an embodiment of thepresent invention.

FIG. 9 is a schematic illustration of optical paths of reference lightbeams in an optical switch in accordance with another embodiment of thepresent invention.

FIG. 10 is a schematic illustration of optical paths of reference lightbeams in an optical switch in accordance with another embodiment of thepresent invention.

FIG. 11 is a timing diagram showing the relative timing of light pulsesprovided by three lasers in accordance with an embodiment of the presentinvention.

FIG. 12 illustrates, in a flow chart, a method of calibrating andaligning an optical switch in accordance with an embodiment of thepresent invention.

FIG. 13 illustrates, in a flow chart, a method of operating an opticalswitch in accordance with an embodiment of the present invention.

FIG. 14 illustrates, in a flow chart, a method for recalibrating anoptical switch in accordance with an embodiment of the presentinvention.

FIG. 15 illustrates, in a block diagram, relationships between mirrorarrays, light sources, and position sensing detectors in an opticalswitch in accordance with an embodiment of the present invention.

Like reference numbers in the various figures denote same parts in thevarious embodiments. Dimensions in the figures are not necessarily toscale.

DETAILED DESCRIPTION

An optical fiber cross-connect switch in accordance with embodiments ofthe present invention routes light input through any one of N inputports to any one of P output ports. In a typical optical path through aswitch, light entering the switch through an input port is incident on acorresponding first micro-mechanical mirror in a first two dimensionalarray of micro-mechanical mirrors. The first micro-mechanical mirror,which can be oriented in a range of arbitrary directions (dθ,dφ), istilted to direct the light to a second micro-mechanical mirror in asecond two dimensional array of micro-mechanical mirrors. The secondmicro-mechanical mirror, which can also be oriented in a range ofarbitrary directions (dθ,d∠), is tilted to direct the light to acorresponding output port and hence out of the switch.

The light may be switched from the output port to which it is initiallydirected to another output port by reorienting the firstmicro-mechanical mirror to direct the light to a third micro-mechanicalmirror in the second array of micro-mirrors, and orienting the thirdmicro-mechanical mirror to direct the light to its corresponding outputport. A control system is provided to control the orientations of themicro-mechanical mirrors and thus accomplish the switching. A number ofembodiments will be described in which the orientations of the variousmicro-mechanical mirrors are measured and controlled by reflecting lightbeams (separate from those being routed through and switched by theoptical fiber switch) from the micro-mechanical mirrors and measuringthe locations at which the reflected light beams are incident onposition sensing detectors.

Although the number of input ports equals the number of output ports(N=P) in the embodiments described below, in other embodiments N<P orN>P.

For convenience of illustration, an optical fiber cross-connect switch 2(FIG. 1) in accordance with one embodiment will be described withreference to N=5 input optical fibers 4 a-4 e serving as input ports,P=N=5 output optical fibers 6 a-6 e serving as output ports, and N=5 ofvarious other optical elements and light beams located in optical pathsbetween input fibers 4 a-4 e and output fibers 6 a-6 e, as shown inFIG. 1. It should be understood, however, that in other embodiments inaccordance with the present invention, N and P are both typicallygreater than about 1000. In one embodiment, for example, N is about 1200and P=N.

As is conventional in Dense Wavelength Division Multiplexing, each ofthe input optical fibers may carry light having a plurality ofwavelengths. In one implementation, the light carried by the inputoptical fibers has wavelengths near about 1310 nanometers (nm) or about1550nm. The input optical fibers and the output optical fibers are, forexample, conventional Corning, Incorporated SMF-28 single mode opticalfibers having a core diameter of about 8 microns (μm) and a claddingdiameter of about 125μm. Other optical fibers suitable for opticalcommunications applications may also be used.

Referring to FIG. 1, N input optical fibers 4 a-4 e enter input fiberblock 8, which rigidly positions their respective ends 10 a-10 e in atwo dimensional array at surface 12 of block 8. Surface 12 is polished,for example, to a flatness better than about 300 nanometers (nm) to forma planar surface including fiber ends 10 a-10 e. Input fiber block 8fixes the positions and orientations of fiber ends 10 a-10 e withrespect to other optical elements in optical switch 2, therebypreventing motion of fiber ends 10 a-10 e from misaligning switch 2.

In one implementation, input fiber block 8 includes a silicon platethrough which pass N substantially parallel holes arranged, for example,in a rectangular array having horizontal and vertical pitches of about 1millimeter (mm). Into each hole is inserted a corresponding one of inputoptical fibers 4 a-4 e. Surface 12 of input fiber block 8 is polished toform a planar surface substantially perpendicular to the input opticalfibers and including their ends 10 a-10 e. At surface 12 the N opticalfiber ends form, for example, a rectangular array as described above.For N=1200, for example, the array may be a rectangular 30×40 array.Various implementations of input fiber block 8 are described in greaterdetail in U.S. Patent Application Attorney Docket No. M-11501 US,incorporated herein by reference.

Light carried by input optical fibers 4 a 4 e is output as divergingcones of light by, respectively, the cores of fiber ends 10 a-10 e. TheN lenslets (small lenses) 14 a-14 e of lenslet array 14 collect lightoutput, respectively, by fiber ends 10 a-10 e and form, respectively, Nsubstantially parallel or weakly focused light beams 16 a-16 e. Lensletarray 14 is located adjacent to input fiber block 8, with lenslets 14a-14 e arranged in a pattern matching that of fiber ends 10 a-10 e suchthat lenslets 14 a-14 e are located with their optical axesapproximately centered on, respectively, fiber ends 10 a-10 e.

In one implementation, lenslets 14 a-14 e have focal lengths of about 4mm, diameters of about 1 mm, and are spaced approximately 3 mm fromsurface 12 of input block 8. Lenslet array 14 is formed, for example,from fused silica, optical glass, silicon, plastic, or epoxy. Suitablelenslet arrays are available, for example, from Adaptive OpticsAssociates, Inc. of Cambridge, Mass., Advanced Micro Optical Systems,GmbH, of Saarbrucken, Germany, and Corning Rochester PhotonicsIncorporated of Rochester, N.Y.

Light beams 16 a-16 e formed by lenslets 14 a-14 e are incident on micromirror array 18. Micro mirror array 18 includes N micro mirrors 18 a-I 8epositioned above reference surface 19 and arranged in a pattern, suchas a rectangular array, corresponding to the patterns of fiber ends 10a-10 e and lenslets 14 a-14 e. The pitch of micro mirrors 18 a-18 e, ina direction along surface 19 parallel to a plane of incidence defined byone of light beams 16 a-16 e and an axis normal to surface 19 at thepoint at which the light beam would intersect surface 19, is elongatedcompared to the corresponding pitch of lenslets 14 a-14 e such thatsubstantially parallel or weakly focused light beams 16 a-16 e areincident approximately centered on micro mirrors 18 a-18 e,respectively. The orientations of micro mirrors 18 a-18 e with respectto reference surface 19 are individually controllable over a range ofarbitrary angles (dθ,dφ) by control system 20 with electrical signalstransmitted via bus 22.

In one embodiment, micro mirror array 18 is a micro-electro-mechanicalsystem (MEMS) micro mirror array described U.S. patent application Ser.No. 09/779,189, incorporated herein by reference. In this embodiment,micro mirrors 18 a-18 e are freely rotatable micro-electro-mechanicalmirrors actuated by, for example, electrostatic, electromagnetic,piezoelectric, or thermal actuation means incorporated into the micromirror array. Other types of micro mirrors and micro mirror arrays mayalso be used. Control system 20 is, for example, an optical switchcontrol system described in U.S. Patent Application Attorney Docket No.M-11419 US and U.S. Patent Application Attorney Docket No. M-11502 US,both of which are incorporated herein by reference.

In one implementation, light beams 16 a-16 e are incident on micromirror array 18 at an angle of incidence of less than about25°, asconventionally defined with respect to an axis normal to mirror arrayreference surface 19, and the footprints of light beams 16 a-16 e (beamspots) on, respectively, micro mirrors 18 a-18 e are consequently nearlycircular. In this implementation, the angles of incidence of the lightbeams on individual micro mirrors may vary, for example, from about 15°to abut 35°. In other implementations, the light beams may be incidenton micro mirror array 18 at an angle of incidence of, for example about0° to about 60°.

The beams spots at the micro mirrors may have nearly uniform intensityor, for example, an intensity that varies radially. One of ordinaryskill in the art will recognize that the diameters of beam spots havingradially varying intensities may be defined in standard mannersdepending on the form of the intensity distribution. For example, thediameter of a Gaussian beam spot having a radial distributionI(r)=I(0)exp(−2(r/w)²) is typically taken to be 2w.

In one implementation, the beam spots at the micro mirrors are nearlycircular with diameters of about 0.6 mm. Small, nearly circular beamspots allow the use of small, nearly circular micro mirrors 18 a-18 e.This is advantageous, as the smaller the micro mirror, the lower itsinertia and the easier it is to move. In addition, nearly circularmirrors experience a more uniform stress from any coating applied tothem and thus deform less than do similarly coated substantiallynoncircular mirrors. Micro mirrors 18 a-18 e are, for example,approximately elliptical with major and minor diameters of about 1.0 mmand 0.9 mm, respectively.

Micro mirrors 18 a-18 e reflect incident light beams 16 a-16 e,respectively, onto fold mirror 24. Fold mirror 24 is, in oneimplementation, a conventional flat gold coated mirror highly reflective(>98%) at infrared wavelengths including 1310 nm and 1550 nm. Suchconventional gold coated mirrors may be obtained from many commercialsources. In another implementation of the illustrated embodiment, foldmirror 24 is a flat dichroic beam splitter that transmits about 30% toabout 70%, preferably about 50%, of incident light having a wavelengthof about 600 nm to about 1000 nm, and reflects greater than about 90%,preferably greater than about 98%, of incident infrared light having awavelength of about 1200 nm to about 1700 nm. Such dichroic mirrors maybe obtained from numerous vendors.

The reflectivity of such a dichroic beam splitter 24 is typicallyselected to allow at least partial separation of wavelengths of lightused in telecommunications (e.g., 1200 nm-1700 nm) from another range(e.g., 600 nm-1000 nm) of non-telecommunication wavelengths used bycontrol light beams described below. In some embodiments thereflectivity of such a dichroic beam splitter 24 atnon-telecommunication wavelengths used by control beams is about 5% toabout 95%.

Fold mirror 24 reflects incident light beams 16 a-16 e onto micro mirrorarray 26. Micro mirror array 26 includes N micro mirrors 26 a-26 epositioned above reference surface 28. The orientations of micro mirrors26 a-26 e with respect to reference surface 28 are individuallycontrollable by control system 20 with electrical signals transmittedvia bus 32. In one implementation, micro mirror arrays 18 and 26 aresubstantially identical.

Each of micro mirrors 18 a-18 e is controllable to reflect a light beamincident on it from input fiber block 8 to any one of micro mirrors 26a-26 e via fold mirror 24. Hence, control system 20 can control theorientations of micro mirrors 18 a-18 e to reflect, via fold mirror 24,any one of light beams 16 a-16 e onto the approximate center of any oneof micro mirrors 26 a-26 e. For example, although FIG. 1 shows lightbeam 16 a reflected by micro mirror 18 a to micro mirror 26 avia foldmirror 24, light beam 16 a could alternatively be reflected by micromirror 18 a to any one of micro mirrors 26 b-26 e via fold mirror 24.Consequently, light beams 16 a-16 e are not necessarily substantiallyparallel to one another between micro mirror array 18 and micro mirrorarray 26.

Control system 20 controls the orientations of micro mirrors 26 a-26 eto reflect the light beams incident on them from micro mirror array 18onto the approximate centers of, respectively, N lenslets 34 a-34 e oflenslet array 34. FIG. 1 shows micro mirrors 26 a-26 e reflectingincident light beams 16 a-16 e to, respectively, lenslets 34 a-34 e. Itshould be understood, however, that each particular one of micro mirrors26 a-26 e is controlled to reflect whichever one of light beams 16 a-16e is incident on it to the lenslet 34 a-34 e corresponding to thatparticular micro mirror. For example, micro mirror 26 a is controlled toreflect whichever one of light beams 16 a-16 e is incident on it tolenslet 34 a.

Micro mirrors 26 a-26 e are positioned above surface 28 in a patterncorresponding to the pattern of lenslets 34 a-34 e with a pitchelongated in one direction such that light beams 16 a-16 e aresubstantially parallel or weakly focused between mirror array 26 andlenslet array 34. In one implementation, light beams 16 a-16 e arereflected from mirror array 26 to lenslet array 34 at an angle ofreflection of less than about 25° as conventionally defined with respectto an axis normal to surface 28. Consequently, in this implementationthe footprints of light beams 16 a-16 e on micro mirrors 26 a-26 e arenearly circular with, for example, beam spot diameters of about 0.6 mm.In this implementation, the angles of reflection of the light beams fromthe individual micro mirrors may vary, for example, from about 15° toabut 35°. In other implementations, the light beams are reflected frommicro mirror array 26 at an angle of reflection of, for example about 0to about 60°.

Lenslet array 34 is located adjacent to surface 36 of output fiber block38. Lenslet array 34 is, for example, substantially identical to lensletarray 14.

Output fiber block 38 rigidly positions the N ends 40 a-40 e of outputfibers 6 a-6 e in a two dimensional array at surface 36. Surface 36 ispolished to form a planar surface including optical fiber ends 40 a-40e. Output fiber block 38 fixes the positions of fiber ends 40 a-40 ewith respect to other optical elements in optical switch 2, therebypreventing motion of fiber ends 40 a-40 e from misaligning opticalswitch 2. Output fiber block 38 is, for example, substantially identicalto input fiber block 8.

Lenslets 34 a-34 e are arranged in a pattern matching that of fiber ends40 a-40 e such that lenslets 34 a-34 e are located with their opticalaxes approximately centered on, respectively, fiber ends 40 a 40 e.Lenslets 34 a-34 e focus light beams reflected by, respectively, micromirrors 26 a-26 e into, respectively, the cores of optical fiber ends 40a-40 e to complete the connections from input ports to output ports.

Optical fiber switch 2 may be symmetrical. In one symmetricalembodiment, for example, fiber blocks 8 and 38 are substantiallyidentical and symmetrically located with respect to fold mirror 24,lenslet arrays 14 and 34 are substantially identical and symmetricallylocated with respect to fold mirror 24, and mirror arrays 18 and 26 aresubstantially identical and symmetrically located with respect to foldmirror 24. The optical paths of light beams 16 a-16 e may besubstantially symmetrical in a symmetrical embodiment in which lightbeam,s 16 a-16 e are focused approximately midway between input fiberends 10 a-10 e and output fiber ends 40 a-40 e.

In one symmetrical embodiment, for example, light beams 16 a-16 e havebeam spot diameters of approximately 0.8 mm at lenslet arrays 14 and 34,approximately 0.6 mm at mirror arrays 18 and 26, and approximately 0.5mm at fold mirror 24. Such symmetrical embodiments allow bi-directionaltransmission of light through optical fiber switch 2 with minimaloptical power loss, i.e., either ports 4 a-4 e or ports 6 a-6 e may beinput ports. Moreover, symmetrical embodiments require fewer componenttypes than do unsymmetrical embodiments and are consequently easier andless expensive to construct.

The size of the optical elements in optical fiber switch 2 are typicallychosen to be small in order to allow the switch to fit, for example,into a small rack space and to minimize optical power loss in theswitch.

The paths of light beams 16 a-16 e resulting from the foldedconfiguration of mirror array 18, fold mirror 24, and mirror array 26 inthe embodiment shown in FIG. 1 are optically equivalent to the paths oflight beams 16 a-16 e resulting from an unfolded configuration in whichthe location of mirror array 26 is reflected through the plane of foldmirror 24, and fold mirror 24 is removed. For example, FIG. 2 shows anembodiment having an unfolded configuration equivalent to the foldedconfiguration of the embodiment shown in FIG. 1. In the embodiment shownin FIG. 2, each of micro mirrors 18 a-18 e is controllable to reflect alight beam incident on it from input fiber block 8 directly to any oneof micro mirrors 26 a-26 e.

In the embodiment shown in FIG. 2, mirror arrays 18 and 26 are orientedsubstantially parallel to each other. Thus, in the optically equivalentembodiment shown in FIG. 1, the relative orientation of mirror arrays 18and 26 is optically equivalent to substantially parallel. In unfoldedembodiments in which micro mirror arrays 18 and 26 are substantiallyparallel, and in the optically equivalent folded embodiments, themaximum swing angle by which any one of micro mirrors 18 a-18 e and 26a-26 e must be tilted with respect to surfaces 19 and 28, respectively,to direct one of light beams 16 a-16 e to one of output fibers 6 a-6 eis reduced compared to embodiments having other optical configurations.In one implementation in which mirror arrays 18 and 26 each includeabout 1200 mirrors, for example, the maximum angle by which any one ofmicro mirrors 18 a-18 e and 26 a-26 e must be tilted is less than about10°. Switching time is consequently reduced compared to configurationsrequiring larger swing angles. Moreover, the maximum optical pathdifference occurring when one of light beams 16 a-16 e is switched fromone to another of output fibers 6 a-6 e is correspondingly reduced toless than about 1 centimeter. Consequently, light beams 16 a-16 e havesimilar divergences and diameters at, for example, mirror array 26 andlenslet array 34 and thus experience similar attenuation. Thus, couplingloss variation between light beams 16 a-16 e is small.

Optical fiber switch 2 may also include a beam splitter located tosample light entering optical switch 2 through input fibers 4 a-4 e.Such sampling allows the intensities of the input light to be monitored,for example. Referring to FIG. 3, in one embodiment optical fiber switch2 includes beam splitter 42 located to reflect portions of light beams16 a-1 6 e, respectively, to form N light beams 44 a-44 e. In oneimplementation, beam splitter 42 is a cube beam splitter formed from BK7 optical glass and having a dielectric coating with a reflectivity ofabout 2% at infrared wavelengths of about 1200 nm to about 1700 nm. Inanother implementation, beam splitter 42 is a dichroic cube beamsplitter formed from BK 7 optical glass and having a dielectric coatingwith a reflectivity of about 2% at infrared wavelengths of about 1200 toabout 1700 and a reflectivity of about 40% to about 60 %, preferablyabout 50%, at wavelengths of about 600 nm to about 1000 nm. Such beamsplitters are available, for example, from Harold Johnson OpticalLaboratories, Inc. of Gardena, Calif. Suitable coatings for the beamsplitter may be obtained, for example, from ZC&R Coatings For Optics,Inc. of Torrance, Calif.

Light beams 44 a 44 e are reflected by conventional 90° prism 46 (or aconventional mirror), present to provide a convenient optical path, toconventional field lens 48. Conventional field lens 48 collects lightbeams 44 a-44 e and directs them onto N lenslets 50 a-50 e,respectively, of lenslet array 50. In one implementation, the focallength of field lens 48 is about 50 mm and the focal length of lenslets50 a-50 e is greater than about 50 mm. The focal length of thecombination of lens 48 and lenslets 50 a-50 e is less than about 50 mmin this implementation. Lenslet array 50 is, for example, a lensletarray similar to lenslet arrays 14 and 34 and available from the samesources.

Lenslets 50 a-50 e focus light beams 44 a-44 e to N separate spots oninput sensor 52, located adjacent to lenslet array 50. Input sensor 52,which has at least N pixels, detects the intensity of each of lightbeams 44 a 44 e to monitor the input signal level to input opticalfibers 4 a-4 e. Input sensor 52 may provide electrical signalscorresponding to the detected intensities to control system 20 via bus53. Thus, input sensor 52 allows input signal presence and leveldetection for each of input optical fibers 4 a-4 e. In oneimplementation, input sensor 52 is a model SU128-1.7RT infrared camerahaving a 128×128 pixel array available from Sensors Unlimited, Inc. ofPrinceton, N.J.

Optical fiber switch 2 may also include a beam splitter located tosample light reflected from fiber ends 40 a-40 e or fiber block 38. Suchsampling allows the intensities of the reflected light to be monitored.The measured reflected intensities can be used in a method for aligningoptical fiber switch 2. In the embodiment shown in FIG. 3, for example,optical fiber switch 2 includes beam splitter 54 located to form N lightbeams 56 a-56 e from portions of light beams 16 a-16 e reflected byoutput fiber block 38 or fiber ends 40 a-40 e. Beam splitter 54 is, forexample, substantially identical to beam splitter 42. Light beams 56a-56 eare reflected by conventional 90° prism 58 (or a conventionalmirror), present to provide a convenient optical path, to conventionalfield lens 60. Conventional field lens 60 collects light beams 56 a-56 eand directs them onto N lenslets 62 a-62 e, respectively, of lensletarray 62. Field lens 60 and lenslet array 62 are, for example,substantially identical to, respectively, field lens 48 and lensletarray 50.

Lenslets 62 a-62 e focus light beams 56 a-56 e to N separate spots onoutput sensor 64, located adjacent to lenslet array 62. Output sensor64, which has at least N pixels, detects the intensity of each of lightbeams 56 a-56 e and provides corresponding electrical signals to controlsystem 20 via bus 53. The intensities of light beams 56 a-56 e depend,respectively on how well aligned the light beams reflected by micromirrors 26 a-26 e are with, respectively, input fiber ends ⁴ 0 a-40 e.The intensity of light beam 56 a, for example, is at a local minimumwhen the light beam reflected by micro mirror 26 a is focused by lenslet34 a to the approximate center of the core of fiber end 40 a, andincreases if the light beam is directed to focus on the cladding offiber end 40 a or on output fiber block 38 near fiber end 40 a (thecladding and the fiber block are more reflective than the fiber core).Thus, if light beams 16 a-16 e are known to be otherwise well aligned(from diagnostics discussed below), minimized reflected intensitiesverify that output signals are present on output fibers 6 a-6 e.Moreover, the intensity of a reflected light beam is also high if thecorresponding output optical fiber is broken. Thus, output sensor 64allows output fiber fault detection.

In one embodiment, Q of the N input optical fibers and Q of the P outputoptical fibers are monitor channels dedicated to monitoring theperformance of optical switch 2. In one implementation, for example, 9of 1200 input optical fibers and 9 of 1200 output optical fibers aremonitor channels. Typically, the monitor channels are distributedapproximately uniformly in the arrays of fibers formed by input fiberblock 8 and output fiber block 38. For example, nine monitor channelsmay be distributed among four corners, four edges, and the center of arectangular array of input or output optical fibers. The spatialdistributions of the input and output monitor channels need not match.

In the embodiment shown in FIG. 4, for example, optical fibers 4 a and 4eand optical fibers 6 a and 6 e are monitor channels. Of course, otherchoices for monitor channels may also be made. Light beams 16 a and 16e, which are monitor light beams in this embodiment, are formed,respectively, from light output by lasers 68 a and 68 e and coupled by,respectively, conventional fiber couplers 66 aand 66 e to,-respectively,input optical fibers 4 a and 4 e. Detectors 72 a and 72 e are coupledby, respectively, fiber couplers 66 a and 66 e to, respectively, opticalfibers 4 a and 4 e to measure the intensity of light coupled into thosefibers from, respectively, lasers 68 a and 68 e. Detectors 72 a and 72 eprovide electrical signals corresponding to the light intensities theymeasure to control system 20 via bus 53.

Monitor light beams 16 a and 16 e may be directed with micro mirrors 18a, 1.8 e, 26 a, and 26 e to either of (monitor channel) optical fibers 6a and 6 e. Detectors 78 a and 78 e are coupled by, respectively,conventional fiber couplers 74 a and 74 eto, respectively, opticalfibers 6 a and 6 e to detect the intensity of light output by one oflasers 68 a and 68 e and coupled into those optical fibers. Detectors 78a and 78 e provide electrical signals corresponding to the lightintensities they measure to control system 20 via bus 53.

Alternatively, monitor light beams may be formed, respectively, fromlight output by lasers 76 a and 76 e coupled by, respectively,conventional fiber couplers 74 a and 74 e to, respectively, opticalfibers 6 a and 6 e. Detectors 80 a and 80 e are coupled by,respectively, fiber couplers 74 a and 74 e to, respectively, opticalfibers 6 a and 6 e to measure the intensity of light coupled into thosefibers from, respectively, lasers 76 a and 76 e. Detectors 80 a and 80 eprovide electrical signals corresponding to the light intensities theymeasure to control system 20 via bus 53.

Monitor light beams formed using lasers 76 a and 76 e may be directedwith micro mirrors 18 a, 18 e, 26 a, and 26 e to either of opticalfibers 4 a and 4 e. Detectors 70 a and 70 e are coupled by,respectively, conventional fiber couplers 66 a and 66 e to,respectively, optical fibers 4 a and 4 e to detect the intensity oflight output by one of lasers 76 a and 76 e and coupled into thoseoptical fibers. Detectors 70 a and 70 e provide electrical signalscorresponding to the light intensities they measure to control system 20via bus 53.

Thus, control system 20 may determine the attenuation of monitor lightbeams during their passage through optical switch 2. Measurements ofsuch attenuation are used in a recalibration method discussed below.

Lasers 68 a, 68 e, 76 a, and 76 e are, for example, conventionalsemiconductor laser diodes capable of operating at wavelengths of about1310 nm or about 1550 nm. Detectors 70 a, 70 e, 72 a, 72 e, 78 a, 78 e,80 a and 80 e are, for example, conventional InGaAs photodiodes capableof detecting light output by lasers 68 a, 68 e, 76 a, and 76 e.

The orientation of mirrors 18 a-18 e may be measured and controlledwithout the use of light beams 16 a-1 6 e. Referring to FIG. 5, forexample, in one embodiment, about 300 milliwatts (mW) of light having awavelength of about 660 nm is output by laser 82, collimated by lensgroup 84 to form light beam 86, and reflected by a conventional highlyreflective mirror 88 onto dichroic beam splitter 24. Other embodimentsmay use lasers operating at other non-telecommunication wavelengths suchas at 810 nm, for example. Typically, the output power of the laser ischosen to provide an adequate signal to noise ratio for measurementsusing position sensing detectors described below. Approximately 50% oflight beam 86 is transmitted by dichroic beam splitter 24 as light beam86 a incident on mirror array 18 with a beam width sufficient toilluminate all of micro mirrors 18 a-18 e. The path of the approximately50% of light beam 86 reflected by dichroic beam splitter 24 is describedbelow with reference to FIG. 6. Laser 82 is, for example, a conventionallaser diode. Suitable laser diodes are available, for example, fromSemiconductor Laser International Corporation of Binghamton, N.Y. andfrom SDL, Inc. of San Jose, Calif. Lens group 84 has, for example, afocal length of about 80 to about 100 millimeters.

Micro mirrors 18 a-18 e (FIG. 5) reflect portions of light beam 86 a toform N control light beams 90 a-90 e incident on dichroic beam splitter42. Dichroic beam splitter 42 reflects light beams 90 a-90 e toconventional beam splitter 92, which reflects about 50% of each of lightbeams 90 a-90 e to lens group 94. Lens group 94 focuses light beams 90a-90 e onto N apertures 96 a-96 e of aperture plate 96. Aperture plate96 is, for example, formed from sheet metal and is about 0.3 mm thick.Apertures 96 a-96 e which are, for example, circular with a diameter ofabout 0.5 mm, are arranged in a pattern corresponding to that of micromirrors 18 a-18 e, such as a rectangular array with a pitch of about 1mm. Light beams 90 a-90 epass through apertures 96 a-96 e, which removetheir diffracted edges, and are incident on N corresponding positionsensing detectors 98 a-98 e included in position sensing detector array98 located behind aperture plate 96. Position sensing detectors 98 a-98e are arranged in a pattern corresponding to that of micro mirrors 18a-18 e, such as a rectangular array with a pitch of about 1 mm.

Position sensing detector array 98 is, for example, a two dimensionalarray of quadrant cell photodiodes bonded to a glass wafer. In oneimplementation, the cells are electrically isolated from each other bysawing or dicing the array after bonding to the glass wafer. Suitablequadrant cell photodiode arrays are available, for example, from UDTSensors, Inc. of Hawthorne, Calif. and from Pacific Silicon Sensor, Inc.of Westlake Village, Calif.

Lens group 94 is a conventional lens group chosen to have a tiltedobject plane located about coincident with surface 19 of mirror array 18and a tilted image plane located about coincident with aperture plate96. Lens group 94 images micro mirrors 18 a-18 e onto aperture array 96with about 1:1 magnification. The images of micro mirrors 18 a-18 e(particularly their centroids) at aperture array 96 are stationary,i.e., they do not substantially move when micro mirrors 18 a-18 e areangularly displaced (tilted) with respect to surface 19. The images arestationary at aperture plate 96 because all rays of light originatingfrom a point in the object plane of lens group 94 and passing throughlens group 94 are focused approximately to a corresponding point in theimage plane. Hence, angular displacements of micro mirrors 18 a-18 echange the paths taken by light beams 90 a-90 e through lens group 94without changing the locations at aperture plate 96 at which micromirrors 18 a-18 e are imaged. However, since position sensing detectorarray 98 is located behind the image plane of lens group 94, the imagesof micro mirrors 18 a-18 e on detector array 98 are displaced in theplane of array 98 when the corresponding micro mirrors are angularlydisplaced. Hence, the orientations of micro mirrors 18 a-18 e can bedetermined from the positions of light beams 90 a-90 e measured by,respectively, position sensing detectors 98 a-98 e. Position sensingdetectors 98 a-98 e provide electrical signals indicating the positionsof light beams 90 a-90 e to control system 20 via bus 53.

The linear displacements of light beams 90 a-90 e on detectors 98 a-98 edue to angular displacements of micro mirrors 18 a-18 e increase as theseparation between position sensing detector array 98 and the imageplane of lens group 94 is increased. Thus, the range over which theorientations of micro mirrors 18 a-18 e are measured can be varied byvarying the position of detector array 98 with respect to aperture plate96. In one implementation, detector array 98 is located parallel to andabout 0.5 mm to about 1 mm behind the image plane of lens group 94 (oraperture plate 96), and light beams 90 a-90 e have beam spot diametersof about 0.5 mm at detectors 98 a-98 e. Detectors 98 a-98 e are eachabout 1 mm square. In this implementation, linear displacements of about±0.25 mm (the maximum without shifting the beam spots off of thedetectors) with respect to the centers of detectors 98 a-98 e correspondto angular displacements of light beams 90 a-90 e by about ±20°, and ofmicro mirrors 18 a-18 e by about ±10°. If the dynamic range of detectors98 a-98 e (determined by the incident optical power) is 12 bits, as istypical, then the orientation of light beams 90 a-90 e can bedetermined, in principle, with a resolution of about 0.01° (40°/4096).Such an angular resolution requires detectors 98 a-98 e to resolvelinear displacements of about 0.12 microns, however. In practice, theorientations of light beams 90 a-90 e are typically determined with aresolution of about 0.08°, and thus to about 9 bit accuracy.Consequently, the electrical signals provided by detectors 98 a-98 eallow control system 20 to control the orientations of micro mirrors 18a-18 e with a resolution of about 0.04° (about 9 bit accuracy).

It should be noted that the orientations of micro mirrors 18 a-18 e maybe measured and controlled to a resolution of about 0.04° without theuse of mirror 88, dichroic beam splitter 42, and beam splitter 92, whichare present to provide a convenient optical path for light beams 86 aand 90 a-90 e.

The orientations of mirrors 26 a-26 e may be similarly measured andcontrolled without the use of light beams 16 a-16 e. Referring to FIG.6, for example, in one embodiment approximately 50% of light beam 86,formed as described above with reference to FIG. 5, is reflected bydichroic beam splitter 24 to conventional highly reflective mirror 99,which reflects it back to dichroic beam splitter 24. Approximately 50%of the light reflected onto dichroic beam splitter 24 by mirror 99 istransmitted by fold beam splitter 24 as light beam 86 bincident onmirror array 26.

Micro mirrors 26 a-26 e reflect portions of light beam 86 b to form Ncontrol light beams 10Oa-100 e incident on dichroic beam splitter 54.Dichroic beam splitter 54 reflects light beams 100 a-100 e toconventional beam splitter 102, which reflects about 25% of each oflight beams 100 a-100 e to lens group 104. Lens group 104 focuses lightbeams 100 a-100 e onto N apertures 106 a-106 e of aperture plate 106.Aperture plate 106 is, for example, substantially identical to apertureplate 96. Light beams 100 a-100 e pass through apertures 106 a-106 e,which remove their diffracted edges, and are incident on N correspondingposition sensing detectors 108 a-108 e in position sensing detectorarray 108 located behind aperture plate 106. Position sensing detectorarray 108 is, for example, substantially identical to position sensingdetector array 98 and available from the same sources. Position sensingdetectors 108 a-108 e are arranged in a pattern corresponding to that ofmicro mirrors 26 a-26 e, such as a rectangular array with a pitch ofabout 1 mm.

Lens group 104 is a conventional lens group chosen to have a tiltedobject plane located about coincident with surface 28 of mirror array 26and a tilted image plane located about coincident with aperture plate106. Lens group 104 images micro mirrors 26 a-26 e onto aperture plate106 with about 1:1 magnification. Lens group 104 is, for example,substantially identical to lens group 94. The images of micro mirrors 26a-26 e (particularly their centroids) at aperture array 106 do not movewhen micro mirrors 26 a-26 e are angularly displaced with respect tosurface 28. However, the images of micro mirrors 26 a-26 e on detectorarray 108 are displaced in the plane of array 108 when the correspondingmicro mirrors are angularly displaced. Hence, the orientations of micromirrors 26 a-26 e can be determined from the positions of light beams100 a-100 e measured by, respectively, position sensing detectors 108a-108 e. Position sensing detectors 108 a-108 e provide electricalsignals indicating the positions of light beams 100 a-100 e to controlsystem 20 via bus 53.

The range over which the orientations of micro mirrors 26 a-26 e aremeasured is determined similarly to that over which the orientations ofmicro mirrors 18 a-18 e are measured. In particular, since the lineardisplacements of light beams 100 a-100 e on detectors 108 a-108 e due toangular. displacements of micro mirrors 26 a-26 e increase as theseparation between position sensing detector array 108 and the imageplane of lens group 104 (or aperture plate 106) is increased, the rangeover which the orientation of micro mirrors 26 a-26 e are measured canbe varied by varying the position of detector array 108 with respect toaperture plate 106. In one implementation, detector array 108 is locatedparallel to and about 0.5 mm to about 1 mm behind aperture plate 106,and light beams 100 a-100 a have beam spot diameters of about 0.5 mm atdetectors 108 a-108 e, which are each about 1 mm square. In thisimplementation, linear displacements of about ±0.25 mm with respect tothe centers of detectors 108 a-108 e correspond to angular displacementsof light beams 100 a-100 e by about ±200, and of micro mirrors 26 a-26 eby about ±10°. The orientation of light beams 100 a-100 e can bedetermined, in principle, with a resolution of about 0.01° if detectors108 a-108 e have a dynamic range of 12 bits. In practice, theorientations of light beams 100 a-100 e are typically determined with aresolution of about 0.08°, and thus to about 9 bit accuracy.Consequently, the electrical signals provided by detectors 108 a-108eallow control system 20 to control the orientations of micro mirrors 26a-26 e with a resolution of about 0.04° (about 9 bit accuracy).

It should be noted that the orientations of micro mirrors 26 a-26 e maybe measured and controlled with a resolution of about 0.04° without theuse of mirror 88, mirror 99, dichroic beam splitter 54, and beamsplitter 102, which are present to provide a convenient optical path forlight beams 86 b and 100 a-100 e.

The electrical signals provided by position sensing detectors 98 a-98 eand 108 a-108 e also allow control system 20 to detect malfunctioningmicro mirrors.

Angular displacements of micro mirrors 18 a-18 e and 26 a-26 e may alsobe measured and controlled with N control light beams (different fromlight beams 16 a-16 e) each of which is reflected from one of micromirrors 18 a-18 e and one of micro mirrors 26 a-26 e. Referring to FIG.7, for example, in one embodiment about 30 mW of light having awavelength of about 660 nm is output by laser 110 and collimated byconventional lens group 112 to form light beam 114 incident on lensletarray 116. Laser 110 is, for example, a conventional laser diode similaror identical to laser 82. Lens group 112 has a focal length of, forexample, about 80 millimeters to about 100 millimeters.

Lenslet array 116 includes N lenslets 116 a-116 e arranged in a patterncorresponding to that of micro mirrors 26 a-26 e, such as a rectangulararray with a pitch of about 1 mm. Lenslets 116 a-116 e, which have focallengths of about 100 mm, for example, form N corresponding substantiallyparallel or weakly focused control light beams 114 a-114 e from portionsof light beam i 14 and focus them to diameters of, for example, about0.16 mm at, respectively, apertures 118 a-118 e of aperture plate 118.Aperture plate 118 is, for example, formed from sheet metal and is about0.3 mm thick. Apertures 118 a-118 e, which are circular with a diameterof about 0.16 mm, for example, are arranged in a pattern matching thatof lenslets 116 a-116 e.

Light beams 114 a-114 e pass through apertures 118 a-118 e, by whichthey are spatially filtered, and are incident on and recollimated by,respectively, N lenslets 120 a-120 e of lenslet array 120. Lenslets 120a-120 e are arranged in a pattern matching that of lenslets 116 a-116 eand have focal lengths, for example, of about 77 mm. Lenslet arrays 116and 120 are, for example, similar to lenslet arrays 14, 34, 50, and 62and available from the same sources.

Conventional 90° prism 122 reflects light beams 114 a-114 e toconventional beam splitter cube 102, which transmits about 75% of eachof light beams 114 a-114 e to dichroic beam splitter 54. Dichroic beamsplitter 54 reflects light beams 114 a-114 e to, respectively, micromirrors 26 a-26 e of mirror array 26. Light beams 114 a-114 e aresubstantially parallel to light beams 16 a-16 e (FIG. 3) betweendichroic beam splitter 54 and mirror array 26.

If micro mirrors 26 a-26 e are approximately oriented to direct lightcarried by input optical fibers 4 a-4 e to output optical fibers 6 a-6 eas described above, then micro mirrors 26 a-26 e reflect light beams 114a-114 e to micro mirrors 18 a-18 e via dichroic beam splitter 24. Itshould be noted that although FIG. 7 shows light beams 114 a-114 edirected, respectively, to micro mirrors 18 a-18 e, each of light beams114 a-114 e may be directed to any one of micro mirrors 18 a-18 e.

If micro mirrors 18 a-18 e are approximately oriented to direct lightcarried by input optical fibers 4 a 4 e to output optical fibers 6 a-6e, then micro mirrors 18 a-18 e reflect whichever ones of light beams114 a-114 e are incident on them from mirror array 26 to dichroic beamsplitter 42 as substantially parallel or weakly focused beams of lightsubstantially parallel to light beams 16 a-16 e (FIG. 3). Dichroic beamsplitter 42 reflects light beams 114 a-114 e to conventional beamsplitter cube 92, which transmits about 50% of each of light beams 114a-114 e to conventional 90° prism 124. Prism 124 reflects light beams114 a-114 e to conventional beam splitter cube 126, which reflects about50% of each of light beams 114 a-114 e to conventional 90° prism 128.Prism 128 reflects light beams 114 a-114 e onto lenslet array 130, whichincludes N lenslets 130 a-130 e arranged in a pattern corresponding tothat of micro mirrors 18 a-18 e, such as a rectangular array with apitch of about 1 mm. Lenslet array 130 is, for example similar to thelenslet arrays described above and available from the same sources.

Lenslets 130 a-130 e focus the particular ones of light beams 114 a-114e reflected by, respectively, micro mirrors 18 a-18 e onto,respectively, position sensing detectors 134 a-134 e of position sensingdetector array 134 located at about the focal plane of lenslet array130. Position sensing detector array 134 is, for example, similar oridentical to position sensing detector arrays 98 and 108 and availablefrom the same sources. Position sensing detectors 134 a-134 e arearranged in a pattern corresponding to that of micro mirrors 18 a-18 e,such as a rectangular array with a pitch of about 1 mm, for example.

Light beams 114 a-114 e are linearly displaced in the plane of detectorarray 134 when the micro mirrors in mirror arrays 18 and 26 from whichthey reflect are angularly displaced. The magnitude of the lineardisplacement of a particular one of light beams 114 a-114 e is aboutequal to the product of the focal length of the lenslet focusing it ontodetector array 134 and the tangent of the beam's angular displacement.Thus, if only one of the micro mirrors from which the beam is reflectedis angularly displaced, the angular displacement of that micro mirrorcan be determined from the linear displacement of the beam on detectorarray 134. Position sensing detectors 134 a-134 e provide electricalsignals indicating the position of light beams 114 a-114 e to controlsystem 20 via bus 53.

The range over which angular displacements of light beams 114 a-114 eare measured is determined by the diameters of the beams at detectors134 a-134 e, the size of detectors 134 a-134 e, and the focal lengths oflenslets 130 a-130 e. In one implementation, for example, the lensletshave focal lengths of 100 mm, the beams have beam spot diameters ofabout 0.5 at detector array 134, and detectors 134 a-134 e are about 1mm square. In this implementation, linear displacements of about ±0.25mm (the maximum without shifting the beam spots off of the detectors)with respect to the centers of the detectors correspond to angulardisplacements of light beams 114 a-114 e by about ±0.15°. If detectors134 a-134 e have a 5 bit dynamic range, which is easily achieved, thenangular displacements of light beams 114 a-114 e can be determined to aresolution of about 0.01° (5 bit accuracy). Consequently, theorientations of micro mirrors 18 a-18 e and 26 a-26 e may be controlledusing measurements made with detectors 134 a-134 e with a resolution ofabout 0.005°. Such an angular resolution requires detectors 134 a-134 eto resolve linear displacements of about 17 microns. Control system 20may thus control the orientations of micro mirrors 18 a-18 e and 26 a-26e with about 12 bit accuracy, since detectors 98 a-98 e and 108 a-108 eallow the micro mirrors to be controlled over a range of about 20°, anddetectors 134 a-134 e allow the micro mirrors to be controlled with aresolution of about 0.0050.

It should be noted that angular displacements of micro mirrors 18 a-18 eand 26 a-26 e may be measured and controlled with a resolution of about0.005° without the use of prism 122, beam splitter 102, beam splitter92, prism 124, beam splitter 126, and prism 128, which are present toprovide a convenient optical path for light beams 114 a-114 e.

The electrical signal provided by a position sensing detector to controlsystem 20 when a control light beam is incident on the detector can varyin time (drift) even if the location at which the control light beam isincident on the detector does not change, i.e., the control light beamdoes not move. Thus, the electrical signal provided by the positionsensing detector can indicate apparent motion of the control light beam(and micro mirrors from which it is reflected) even if no such motionhas occurred, and thereby introduce errors into the measurement andcontrol of the orientations of those micro mirrors. Such detector driftcan be caused, for example, by time varying temperature gradients acrossthe detector which produce time varying spatial gradients in detectorresponsivity.

The effects of such drift can be reduced by the use of a plurality ofreference beams, each of which is incident on the approximate center ofa corresponding one of the position sensing detectors. If the controllight beams and reference light beams are time gated (pulsed) andinterleaved in time, then the position sensing detectors can provideseparate electrical signals indicating the locations at which thereference and control light beams are incident on the detectors. Sincethe electrical signals provided by a position sensing detector inresponse to control and reference light beams drift similarly, adifference signal generated, for example, by subtracting the signalprovided in response to the reference beam from the signal provided inresponse to a control beam can be substantially free of drift.

Accordingly, reference beams may be used to reduce the effects ofdetector drift on the measurement and control of micro mirrors 18 a-18 eand 26 a-26 e. Referring to FIG. 8, for example, in one embodiment about30 mW of light having a wavelength of about 660 nm is output byconventional laser 136 and collimated by conventional lens group 138 toform light beam 140 incident on conventional beam splitter cube 126.Laser 136 is, for example, similar or identical to lasers 82 and 110 andavailable from the same sources.

About 50% of light beam 140 is transmitted by beam splitter 126 as lightbeam 141 to conventional 90° prism 124, which reflects light beam 141 toconventional beam splitter cube 92. Beam splitter cube 92 reflects about50% of light beam 141 to conventional highly reflective mirror 144,which retroreflects it through beam splitter 92 to lens group 94 aslight beam 146. Mirror 144 is, for example, a conventional highlyreflective metal or dielectric coating on a surface of beam splitter 92.

Lens group 94 directs light beam 146 onto aperture array 96. Lens group94 and aperture array 96 were described above with reference to FIG. 5.Apertures 96 a-96 e of aperture array 96 form, respectively, N referencelight beams 146 a-146 e having beam spot diameters of about 0.5 andincident on the approximate centers of, respectively, position sensingdetectors 98 a-98 e of position sensing detector array 98. Positionsensing detectors 98 a-98 e provide electrical signals indicating the(physically stationary) positions of reference light beams 146 a-146 eto control system 20 via bus 53.

Referring to FIG. 9, in one embodiment conventional beam splitter 126reflects about 50% of light beam 140 (formed as described above withreference to FIG. 8) to conventional highly reflective mirror 142, whichretroreflects it through beam splitter 126 as light beam 148 incident onconventional 90° prism dielectric coating on a surface of beam splitter126. Prism 128 reflects light beam 148 to lens array 130, describedabove with respect to FIG. 7. Lenslets 130 a-130 e form, respectively, Nreference beams 148 a-148 e and focus them on, respectively, theapproximate centers of position sensing detectors 134 a-1 34 e.Reference beams 148 a-148 e have, for example, beam spot diameters ofabout 0.5 mm at detectors 134 a-134 e. Position sensing detectors 134a-134 e provide electrical signals indicating the (physicallystationary) positions of reference light beams 148 a-148 e to controlsystem 20 via bus 53.

Referring to FIG. 10, conventional beam splitter 102 reflects about 25%of light beams 114 a-114 e, formed as described above with reference toFIG. 7, to conventional highly reflective mirror 149. Mirror 149 is, forexample, a conventional highly reflective metal or dielectric coating ona surface of beam splitter 102. Mirror 149 retroreflects the portions oflight beams 114 a-114 e incident on it through beam splitter 102 toconventional lens group 104 as reference light beams 150 a-150 e. Lensgroup 104 directs reference light beams 150 a-150 e onto, respectively,apertures 106 a-106 e of aperture array 106. Lens group 104 and aperturearray 106 were described above with reference to FIG. 6.

Reference light beams 150 a-150 e pass through apertures 106 a-106 e andare incident on, respectively, the approximate centers of positionsensing detectors 108 a-108 e with beam spot diameter of, for example,about 0.5 . Position sensing detectors 108 a-108 e provide electricalsignals indicating the (physically stationary) positions of referencelight beams 150 a-150 e to control system 20 via bus 53.

In one embodiment, lasers 82, 110, and 136 each emits pulses of light ofabout 33 microseconds (μs) duration at a repetition rate of about 10kilohertz (kHz). The three trains of light pulses emitted by lasers 82,110, and 136 are interleaved in time to provide to each of the positionsensing detectors an alternating sequence of reference beam and controlbeam light pulses having a repetition rate of about 10 kHz. Hence, theposition sensing detectors each provide an alternating sequence ofelectrical signals in response to the control and reference light beamsat a repetition rate of about 10 kHz. This allows the micro mirrors tobe controlled to switch light input through one of input fibers 4 a-4 eand initially directed to one of output fibers 6 a-6 e to another ofoutput fibers 6 a-6 e in less than about 10 milliseconds (ms).

Referring to the timing diagrams of FIG. 11, for example, laser 82, fromwhich are derived control light beams 90 a-90 e (FIG. 5) and 100 a-100 e(FIG. 6), emits a first train of light pulses at about 100 μs intervals.Laser 110, from which are derived control light beams 114 a-114 e (FIG.7) and reference light beams 150 a-150 e (FIG. 10), emits a second trainof light pulses displaced in time by about +33 μs with respect to thefirst train of light pulses. Laser. 136, from which are derivedreference light beams 146 a-146 e (FIG. 8) and 148 a-148 e (FIG. 9),emits a third train of light pulses displaced in time by about +66 μswith respect to the first train of pulses.

The relationships-between mirror arrays 18 and 26, position sensingdetector arrays 98, 108, and 134, lasers 82, 110, and 136, and controlsystem 20 in embodiments of optical fiber switch 2 are furtherillustrated in the block diagram of FIG. 15. It should be noted that thevarious dashed lines representing light beams in FIG. 15 do not indicatedetailed optical paths. Each mirror array has associated with it aposition sensing detector array and a source of control light beams(e.g., laser 82) which are directed by mirrors in the mirror array tothe position sensing detector without being reflected by mirrors in theother array. This arrangement enables coarse control (e.g., resolutionbetter than about 0.04° ) of the orientations of the mirrors in eachmirror array over large ranges of angles (e.g., greater than about 20°).Although each mirror array in the illustrated embodiments has associatedwith it a separate position sensing detector for coarse control, asingle position sensing detector may be used to detect control beamsdirected to it by both mirror array 18 and mirror array 26.

Finer resolution control of the orientation of the mirrors (e.g.,resolution better than about 0.005°) in the mirror arrays and of thelight beams switched by optical fiber switch 2 over narrower ranges ofangles is enabled by the use of control beams which are directed bymirrors in one mirror array to mirrors on the other mirror array andthence to a position sensing detector array. In combination, thearrangements for coarse and fine control allow fine control of themirror orientations over a large range of angles such as, e.g., aresolution of better than about 0.005° over a range of angles greaterthan about 20°.

In the illustrated embodiments control light is directed to the mirrorarrays through dichroic beam splitter 24 (FIGS. 1, 5, and 6) locatedbetween the mirror arrays in an optical path of the, e.g.,telecommunication light beams to be directed from input ports to outputports. The mirrors in the mirror arrays direct the control beams toposition sensing detector arrays via dichroic beam splitters 42 and 54(FIGS. 1, 5, and 6). In other embodiments however, control light may bedirected to the mirror arrays via dichroic beam splitters positionedsimilarly to dichroic beam splitters 42 and 54. Also, the mirrors in themirror arrays may direct control beams to position sensing detectorsthrough a dichroic beam splitter positioned similarly to dichroic beamsplitter 24.

The illustrated embodiments employ three lasers as sources of controland reference light beams. Other embodiments may employ more or fewerthan three light sources to provide control and reference light beams.Moreover, control and reference light beams provided by the same lightsource in the illustrated embodiments may be provided by different lightsources in other embodiments.

An optical fiber switch in accordance with an embodiment of the presentinvention may be initially aligned with method 200 outlined in theflowchart of FIG. 12. In step 210, the position sensing detectors arecalibrated prior to their installation in the optical fiber switch.Collimated light beams having wavelengths, intensities, and diameterssimilar to those of the control and reference beams described above aredirected onto the position sensing detectors. The electrical signalsproduced in response by the detectors and the locations at which thelight beams are incident on the detectors (independently measured with:a microscope, for example) are recorded in a calibration look-up table.The calibration look-up table is stored, for example, in control system20. This process is repeated about N×P times (where N and P are thenumber of micro mirrors in micro mirror arrays 18 and 26, respectively),with the light beams incident at different locations on the detectorseach time.

It should be noted that calibration look-up tables prepared in step 210are substantially identical for substantially identical arrays ofposition sensing detectors. Hence, it is not necessary to prepareseparate look-up tables for each of position sensing detector arrays 98,108, and 134 if the detector arrays are substantially identical.Moreover, the calibration look-up table may be prepared frommeasurements made with a position sensing detector array (or averagedmeasurements made with several position sensing detector arrays) otherthan those included in optical switch 2.

After the position sensing detectors are calibrated, in step 220infrared light having a wavelength used in telecommunications, forexample, is introduced to optical switch 2 through input optical fibers4 a 4 e to form light beams 16 a-16 eincident on micro mirrors 18 a-18 e(FIG. 1). Next, in step 230, a switch configuration is selectedcorresponding to a desired coupling of input optical fibers 4 a-4 e tooutput optical fibers 6 a-6 e.

After step 230, in step 240 control system 20 approximately aligns micromirrors 18 a-18 e to direct light beams 16 a-16 e to the particular onesof micro mirrors 26 a-26 e consistent with the selected switchconfiguration. Control system accomplishes this by using the informationin the calibration look-up table to control micro mirrors 18 a-18 e todirect control light beams 90 a-90 e to predetermined positions onposition sensing detectors 98 a-98 e corresponding to the requiredorientations of micro mirrors 18 a-18 e (FIG. 5). The orientations ofmicro mirrors 18 a-18 e required to direct light beams 16 a-16 e toparticular ones of micro mirrors 26 a-26 e can be calculated from theknown geometry of optical switch 2.

Next, in step 250 control system 20 approximately aligns micro mirrors26 a-26 e to direct whichever ones of light beams 16 a-16 e are incidenton them to, respectively, lenslets 34 a-34 e and thus approximately tofiber ends 40 a-40 e (FIG. 1). Control system 20 accomplishes this byusing the information in the calibration look-up table to control micromirrors 26 a-26 e to direct control light beams 100 a-100 e to positionson position sensing detectors 108 a-108 e corresponding to the requiredorientations of micro mirrors 26 a-26 e (FIG. 6). The requiredorientations of micro mirrors 26 a-26 e can be calculated from the knowngeometry of optical switch 2 and the known orientations of micro mirrors18 a-18 e.

Typically, each of control light beams 114 a-114 e will be incident on acorresponding one of position sensing detectors 134 a-134 e after step250 is performed (FIG. 7). Light beams 16 a-16 e may be sufficientlymisaligned, however, that lenslets 34 a-34 e do not necessarily focusthe particular ones of light beams 16 a-I 6 e incident on them onto,respectively, the cores of fiber ends 40 a-40 e.

After step 250, in step 260 control system 20 controls micro mirrors 26a-26 e or micro mirrors 18 a-18 e to minimize the intensity of lightbeams 56 a-56 ereflected, respectively, by fiber ends 40 a-40 e orneighboring regions of fiber block 38 and detected by output sensor 64(FIG. 3). The minimum intensity reflections may be found, for example,by raster scanning light beams 16 a-16 e across the particular fiberends to which they were approximately directed in step 250. As a resultof this minimization process, light beams 16 a-1 6 e are focused ontothe cores of the fiber ends upon which they are incident. Thus, afterstep 260 at least a portion of the light in each of light beams 16 a-16e is coupled into the particular one of output optical fibers 6 a-6 e towhich it is directed.

Next, in step 270 control system 20 fine-tunes the alignment of lightbeams 16 a-16 e by varying the orientations of micro mirrors 18 a-18 eand 26 a-26 e to maximize the intensity of the light coupled intooptical fibers 6 a-6 e. For example, the intensity of light coupled intooutput optical fiber 6 a may be maximized by iteratively varying theorientation of micro mirror 26 a and the orientation of the one of micromirrors 18 a-18 e from which the light beam incident on fiber 6 a isreflected. The intensity of the light coupled into the output opticalfibers may be measured, for example, with conventional InGaAsphotodiodes temporarily optically coupled to the output optical fibers.As a result of this maximization process, light beams 16 a-16 e areapproximately centered on the micro mirrors by which they are reflectedand approximately centered on the cores of the output fibers upon whichthey are focused.

Next, in step 280 control system 20 calculates and records, in analignment look-up table, differences between the electrical signalsprovided by position sensing detectors 98 a-98 e, 108 a-108 e, and 134a-134 e in response to the control and reference light beams. Thesedifference signals represent positions on the position sensing detectorsand correspond to the optimal alignment of light beams 16 a-16 e. Afterstep 280, at step 290 control system 20 determines whether steps 240through 280 have been performed for all possible connections of inputports to output ports. If not, control system 20 returns to step 230.

Method 200 is one of several methods by which an optical fiber switch inaccordance with an embodiment of the present invention may be initiallyaligned and calibrated. Other suitable methods that may be used inaddition to or in place of method 200 are described in U.S. PatentApplication Attorney Docket No. M-11419 US.

After initial calibration and alignment, control system 20 may operatean optical switch in accordance with an embodiment of the presentinvention with operation method 300 outlined in the flowchart of FIG.13. In step 310, control system 20 selects a switch configurationcorresponding to a desired coupling of input optical fibers 4 a 4 e tooutput optical fibers 6 a-6 e. Next, in step 320, control system 20retrieves from the alignment look-up table the difference signals(positions on the position sensing detectors) corresponding to theoptimized alignment of the selected switch configuration.

After step 320, in step 330 control system 20 aligns micro mirrors 18a-18 e to direct control light beams 90 a-90 e to predeterminedpositions on position sensing detectors 98 a-98 e corresponding to theselected switch configuration. Control system 20 accomplishes this, forexample, by controlling micro mirrors 18 a-18 e such that differencesbetween the electrical signals provided by detectors 98 a-98 e inresponse to control and reference light beams reproduce differencesignals retrieved in step 320. Control system 20 also aligns micromirrors 26 a-26 eto direct control light beams 100 a-100 e topredetermined positions on position sensing detectors 108 a-108 ecorresponding to the selected switch configuration.

Control system 20 accomplishes this, for example, by controlling micromirrors 26 a-26 e such that differences between the electrical signalsprovided by detectors 108 a-108 e in response to control and referencelight beams reproduce difference signals retrieved in step 320.

Next, in step 350 control system 20 aligns either micro mirrors 18 a-18e or micro mirrors 26 a-26 e to direct control light beams 114 a-114 eto predetermined positions on position sensing detectors 134 a-134 ecorresponding to the selected switch configuration. Control system 20accomplishes this, for example, by controlling micro mirrors 18 a-18 eor micro mirrors 26 a-26 e such that differences between the electricalsignals provided by detectors 134 a-134 e in response to control andreference light beams reproduce difference signals retrieved in step320. From step 350, control system 20 returns to step 310.

Method 300 does not require the presence of light beams 16 a-16 e inoptical switch 2. Thus, control system 20 can confirm that micro mirrors18 a-18 e and 26 a-26 e are aligned to couple light output by aparticular one of input optical fibers 4 a-4 e into a particular one ofoutput optical fibers 6 a-6 e even if the particular input optical fiberis not carrying light. That is, control system 20 can provide dark fiberconfirmation. In one embodiment after step 350 control system 20 sends asignal to a network node controller, for example, indicating that aparticular connection between an input port and an output port has beenestablished. This signal may be sent before light is introduced into theinput port.

The relative positions of the various optical elements in optical switch2 may vary with time. Such variations may be due, for example, toexpansion or contraction caused by changes in temperature or tovibrations. Thus, occasionally it may be advantageous to realign andrecalibrate optical switch 2. Optical switch 2 may be realigned andrecalibrated with recalibration method 400 outlined in the flowchart ofFIG. 14. In step 410 control system 20 selects a switch configurationcorresponding to a desired coupling of monitor channels 4 a and 4 e tomonitor channels 6 a and 6 e (FIG. 4). Next, in step 420 control system20 executes the steps of method 300 to make the selected connectionsbetween monitor channels.

Next, in step 430 control system 20 fine-tunes the alignment of monitorbeams 16 a and 16 e by varying the orientations of micro mirrors 18 a,18 e, 26 a, and 26 e to minimize the attenuation of the monitor beamsduring their passage through optical switch 2. After step 430, in step440 control system 20 calculates and stores differences between theelectrical signals provided by position sensing detectors 98 a, 98 e,108 a, 108 e, 134 a, and 134 e in response to control and referencebeams. These difference signals represent positions on the positionsensing detectors and correspond to the reoptimized alignment of monitorlight beams 16 a and 16 e

Next, in step 450 control system 20 determines whether steps 420 through440 have been performed for all monitor channel coupling connections. Ifnot, control system 20 returns to step 410.

If steps 420 through 440 have been performed for all monitor channelconnections, then in step 460 control system 20 determines a systematicshift between the difference signals stored at step 440 and thecorresponding difference signals stored in the alignment look-up tableduring initial alignment and calibration (e.g., factory calibration) ofthe optical switch. Next, in step 470 control system 20 calculatescorrection terms to all of the difference signals stored during theinitial alignment and calibration of the optical switch from the shiftdetermined in step 460. Difference signals to be used by control system20 when executing method 300 are combinations of these correction termswith difference signals stored during initial alignment and calibrationof the optical switch. Next, in step 480 control system 20 stores thecorrection terms in the alignment look-up table.

Variations of and more detailed implementations of method 400 aredescribed in U.S. Patent Application Attorney Docket No. M-11419 US.

An optical fiber switch 2 in accordance with an embodiment of thepresent invention typically operates with an insertion loss of less thanabout 3 decibels. That is, the power in an optical signal carried by oneof input optical fibers 4 a 4 e is typically attenuated by less thanabout 3 decibels during passage through optical fiber switch 2 into oneof output optical fibers 6 a-6 e. This low insertion loss results inpart because the orientations of micro mirrors 18 a-18 e and 26 a-26 ecan be measured and controlled without sampling the light carried by theinput optical fibers. In addition, the precision with which thealignments of light beams 16 a-16 e are controlled results in efficientcoupling of the light beams into the output optical fibers. Moreover,optical fiber switch 2 may be physically compact. Consequently, theoptical paths of light beams 16 a-16 e through optical fiber switch 2are typically less than about 360 mm in length. Hence, the diameters oflight beams 16 a-16 e remain small and losses due to diffraction arethus low.

While the present invention is illustrated with particular embodiments,the invention is intended to include all variations and modificationsfalling within the scope of the appended claims.

1. (Canceled).
 2. The optical switch of claim 35, further comprising afirst light source to direct the first light beam to the first mirror.3. The optical switch of claim 35, wherein the first and the secondmirrors are disposed on a surface tilted with respect to an optical axisof the first and the second light beams, respectively.
 4. The opticalswitch of claim 35, wherein the first and the second detectors arearranged as a substantially planar array tilted with respect to anoptical axis of the first and the second light beam, respectively. 5.(Canceled).
 6. The optical switch of claim 5, wherein the first and thesecond detectors are arranged as a substantially planar array, andfurther comprising an aperture plate substantially parallel to andspaced apart from said array by about 0.5 to 1.0 millimeters. 7-8.(Canceled).
 9. The optical switch of claim 35, further comprising: afirst light source; and a second light source, wherein said first lightsource and said second light source each output pulses of light suchthat pulses of light output by said first light source do notsubstantially overlap in time at said detectors with pulses of lightoutput by said second light source.
 10. (Canceled).
 11. The opticalswitch of claim 9, wherein said first light source includes a laser.12-34. (Canceled).
 35. An apparatus, comprising: an optical switchhaving: a first mirror to receive a first light beam and to reflect thefirst light beam to a predetermined position on a first position sensingdetector; a second mirror to receive a second light beam and to reflectthe second light beam to a predetermined position on a second positionsensing detector; a control system to control a first orientation of thefirst mirror, the first mirror to reflect the first light beam to apredetermined position on a first position sensing detector in responseto the orientation, the first mirror to receive a third light beam andto direct the third light beam received from a first port to the secondmirror in response to the first orientation, the control system tocontrol a second orientation of the second mirror, the second mirror toreflect the second light beam to a predetermined position on a secondposition sensing detector in response to the second orientation, thesecond mirror to direct the third light beam to a second port to thesecond mirror in response to the second orientation, the first and thesecond orientations to maximize an intensity of the third light beamcoupled into the second port, the control system to record signalsoutput from the first and second position sensing detectors.
 36. Theapparatus of claim 1, further comprising a first lens and a second lensto focus light onto a corresponding one of the first and seconddetectors, respectively.
 37. The apparatus of claim 1, wherein thecontrol system is to control the first and/or the second orientation ofthe first and/or the second mirror with an angular resolution betterthan about 0.005°.
 38. The apparatus of claim 1, wherein a signaldetermined from an output of the first position sensing detectorsubstantially reproduces a signal determined from a previous output ofthe first position sensing detector.
 39. The apparatus of claim 1,further comprising a look-up table to store a signal determined from anoutput of the first position sensing detector.
 40. The apparatus ofclaim 1, wherein the first port and the second port are a first opticalfiber and a second optical fiber, respectively.
 41. The apparatus ofclaim 40, further comprising a block to fix positions and/ororientations of the first and the second optical fibers with respect tothe first and the second mirrors, respectively.